Mgb2 single crystal and its production method, and superconductive material containing mgb2 single crystal

ABSTRACT

The invention is intended to establish means for manufacturing MB 2  single crystals and to provide a useful superconductive material (wire rod and so forth) taking advantage of anisotropic superconductive properties thereof. A mixed raw material of Mg and B or a precursor containing MgB 2  crystallites, obtained by causing reaction of the mixed raw material of Mg and B, kept in contact with hexagonal boron nitride (hBN), is held at a high temperature in the range of 1300 to 1700° C. and under a high pressure in the range of 3 to 6 GPa to cause reaction for forming an intermediate product, thereby growing the MB 2  single crystals having anisotropic superconductive properties via the intermediate product. The single crystals have features such that, depending on a direction in which a magnetic field is applied thereto, an irreversible magnetic field strength becomes equivalent to not less than 95% of a second magnetic field strength, so that adjustment of crystal orientation thereof results in production of a superconductive material excellent in property. Further, it is useful in effecting growth of the single crystals to cause a reducing agent such as Mg and so forth to coexist at the time of the reaction, or to provide a temperature gradient in melt occurring in the course of the reaction.

TECHNICAL FIELD

[0001] The invention relates to an MgB₂ single crystal havinganisotropic superconductive properties, a superconductive materialcontaining the same, and a method of manufacturing the same, making itpossible to provide a superconducting wire and superconductive thin filmthat are operatable at a relatively high temperature.

BACKGROUND TECHNOLOGY

[0002] Since it has recently been discovered that MgB₂, that is,magnesium boride is a superconductive substance having a relatively highcritical temperature (Tc =about39K), researches from various viewpointshave been conducted in order to elucidate its properties in detail.However, an MgB₂ material has been so far known only in the form of“fine powders” or “polycrystalline bulk material”, and further, therehas been reported no case concerning manufacture of an MgB₂ singlecrystal, so that satisfactory results of the researches have not beengained as yet.

[0003] A reason for difficulty encountered in growing a single crystalof MgB₂ lies in that, upon heating MgB₂ in an attempt to obtain firstMgB₂ melt in order to cause growth of the single crystal thereof, thereoccurs a phenomenon causing MgB₂ to be decomposed into MgB₄ and MgB₆ ata temperature lower than the melting temperature of MgB₂.

[0004] Although there has been a report stating “a trace of MgB₂ thatappeared like crystallites was detected as a byproduct upon synthesizingcubic boron nitride (cBN) from hexagonal boron nitride (hBN) under highpressure” (N. E. Flonenko et al. Dokl Akad. Nauk SSSR 175 (1967), pp.833 to 836), details of the report are unknown, being far fromcontributing to elucidation of the properties of MgB₂ in detail.

[0005] Under the circumstances, it is an object of the invention toestablish means for manufacturing MgB₂ single crystals, thereby openinga way to considerably expand applicable fields of MgB₂ that is highlyhoped for as an excellent superconductive material.

DISCLOSURE OF THE INVENTION

[0006] To that end, the inventor et al. have conducted intensiveresearches, and as a result, the following knowledge has been obtained.

[0007] (a) As previously described, since MgB₂ is caused to bedecomposed into MgB₄ and MgB₆ before MgB₂ is melted if the same in as-isstate is heated at a high temperature, it has been impossible to obtainstable MgB₂ melt. However, if a mixed raw material of Mg and B or MgB₂powders obtained by causing reaction of the mixed raw material of Mg andB, kept in contact with hexagonal boron nitride (hBN), respectively, isheated up under high pressure, this will generate Mg₃BN₃ that is aeutectic composition of Mg, B and N, and so forth, whereupon there occurregions where Mg₃BN₃ and so forth are turned into melt thereof at atemperature lower than the decomposition temperature of MgB₂ in an Mg-B2constituent system, and if this state is kept, there occurscrystallization of MgB₂ crystals from the melt containing Mg₃BN₃ and soforth, thereby causing crystal growth of the MgB₂ crystals withoutoccurrence of decomposition into MgB₄ and so forth.

[0008] (b) In this case, if the MgB₂ powders in non-melted state existin the melt, these will act as nuclei for crystallization of the MgB₂crystals, thereby causing crystal growth thereof, so that it becomespossible to grow relatively large crystals in shorter time.

[0009] (c) Further, if a reducing agent (such as Mg etc. having strongoxidizability) is caused to coexist in a reacting system in such a wayas to be spatially separated from the mixed powders of Mg and B that areraw materials of MgB₂ crystals, the reducing agent will absorb oxygenthat cannot be prevented from being mixed into the reacting system,thereby lowering a partial pressure of oxygen in the reacting system, sothat growth of the MgB₂ crystals is facilitated, resulting in morestable growth of MgB₂ single crystals.

[0010] (d) In addition, if a temperature gradient is caused to occur inMg₃BN3 melt occurring by heating up under high pressure, this willfurther promote the growth of the MgB₂ crystals.

[0011] (e) The MgB₂ single crystals obtained by the above-describedmethod, and so forth, are of a hexagonal structure whereintwo-dimensional boron atom layers and magnesium atom layers arealternately deposited on top of each other in the vertical direction,exhibiting pronounced anisotropy in respect of superconductiveproperties (for example, a second magnetic field Hc₂ in the case where amagnetic field is applied in such a way as to be parallel with boronfaces thereof differs considerably from that in the case where themagnetic field is applied in such a way as to be perpendicular to theboron faces), so that it is possible to manufacture a kind of materialcapable of exhibiting superconductive properties of MgB₂ in the beststate by forming a configuration wherein a plurality of the singlecrystals are bonded in such a way that respective crystal orientationsthereof are aligned with each other.

[0012] (f) The lower the temperature of the MgB₂ single crystals is, orthe higher the purity and crystallinity thereof are, the more pronouncedthe anisotropy in respect of superconductive properties of the MgB₂single crystals becomes, however, with the method described in theforegoing, it is possible to obtain such high-purity MgB₂ singlecrystals as one having “an anisotropy ratio not less than 2.3 at atemperature of 25K”, and in addition, this MgB₂ single crystals have afeature such that an irreversible magnetic field H_(irr), is very closeto a second magnetic field Hc₂ in the case where a magnetic field isapplied thereto in such a way as to be parallel with the boron faces.Taking advantage of the feature, it is possible to manufacture asuperconductive material and so forth that can allow largesuperconducting current to flow even if a high magnetic filed is appliedthereto provided that the high magnetic filed is parallel with the boronfaces, thus enabling applicable fields of MgB2 to be considerablyexpanded.

[0013] Herein the anisotropy ratio described above is defined by thefollowing formula${{anisotropy}\quad {ratio}} = \frac{{Hc}_{2}\left( {H//c} \right)}{{Hc}_{2}\left( {H//{ab}} \right)}$

[0014] (g) Further, it is not that a kind of material capable of havingthe above-described superconductive properties based on the MgB₂ singlecrystals is not necessarily limited to material made up of the MgB₂single crystals only, and any material can exhibit correspondinglyexcellent superconductive properties even if other substances (forexample, Mg, B, and MgB₂ powders that have not reacted as yet) are mixedtherein provided that the MgB₂ single crystals are included therein.

[0015] The invention has been developed on the basis of respective itemsof the knowledge described above, and is intended to provide a MgB₂single crystal superconductor, and methods of manufacturing the same, asdescribed under the following items (1) to (8), respectively.

[0016] (1) An MgB₂ single crystal having anisotropic superconductiveproperties such that a critical magnetic field anisotropy ratio at atemperature of 25K is not less than 2.3, and in the case where amagnetic field is applied thereto so as to be parallel with boron faces,an irreversible magnetic field strength is equivalent to not less than95% of a second magnetic field strength.

[0017] (2) A method of manufacturing MgB₂ single crystals, comprisingthe steps of:

[0018] preparing a mixed raw material of Mg and B; heating and meltingthe mixed raw material, kept in contact with boron nitride (BN), at ahigh temperature in the range of 1300 to 1700° C. and under a highpressure in the range of 3 to 6 GPa; and

[0019] causing growth of the MgB₂ single crystals having anisotropicsuperconductive properties by holding the mixed raw material in theabove-described state.

[0020] (3) A method of manufacturing MgB₂ single crystals, comprisingthe steps of:

[0021] producing a precursor containing MgB₂ crystallites, obtained bycausing reaction of a mixed raw material of Mg and B;

[0022] heating and melting the precursor, kept in contact with hexagonalboron nitride (hBN), at a high temperature in the range of 1300 to 1700°C. and under a high pressure in the range of 3 to 6 GPa; and

[0023] causing growth of the MgB₂ single crystals having anisotropicsuperconductive properties by holding the precursor in theabove-described state.

[0024] (4) The method of manufacturing MgB₂ single crystals, as set outunder the above-described items 2 or 3, wherein in the course ofheating, and melting the raw material or the precursor, to be heated andmelted, at the high temperature and under the high pressure, and holdingthe same in the above-described state, a reducing agent is caused tocoexist therewith.

[0025] (5) The method of manufacturing MgB₂ single crystals, as set outunder any of the above-described items 2 to 4, wherein in the course ofheating and melting the raw material or the precursor, to be heated andmelted, at the high temperature and under the high pressure, and holdingthe same in the above-described state, a temperature gradient of from150 to 300° C. is provided in melt occurring as a result of the heatingand melting the raw material or the precursor.

[0026] (6) The method of manufacturing MgB₂ single crystals, as set outunder any of the above-described items 2 to 5, wherein the MgB₂ singlecrystals have anisotropic superconductive properties which aresuperconductive properties such that a critical magnetic fieldanisotropy ratio at a temperature of 25K is not less than 2.3, and inthe case where a magnetic field is applied thereto so as to be parallelwith boron faces, an irreversible magnetic field strength is equivalentto not less than 95% of a second magnetic field strength.

[0027] (7) A superconductive material comprising MgB₂ single crystalshaving anisotropic superconductive properties such that a criticalmagnetic field anisotropy ratio at a temperature of 25K is not less than2.3, and in the case where a magnetic field is applied thereto so as tobe parallel with boron faces, an irreversible magnetic field strength isequivalent to not less than 95% of a second magnetic field strength.

[0028] (8) A superconductive wire rod comprising MgB₂ single crystalshaving anisotropic superconductive properties such that a criticalmagnetic field anisotropy ratio at a temperature of 25K is not less than2.3, and in the case where a magnetic field is applied thereto so as tobe parallel with boron faces, an irreversible magnetic field strength isequivalent to not less than 95% of a second magnetic field strength.

[0029] As described in the foregoing, with the invention, stable growthof MgB₂ single crystals can be attained by specifying conditions, and soforth, for implementing liquid state of MgB₂, so that the MgB₂ singlecrystals or a superconductive material comprising the MgB₂ singlecrystals, hoped for use in wide application fields, can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is a graph showing optimum conditions for obtaining an MgB₂single crystal;

[0031]FIG. 2 is a schematic illustration showing a manner in which rawmaterials for MgB₂ single crystals according to an embodiment of theinvention are sealed into a vessel made of hBN;

[0032]FIG. 3 is a scanning electron micrograph of the MgB₂ singlecrystal according to the embodiment of the invention;

[0033]FIG. 4 is a schematic diagram of the crystal structure of materialobtained according to the embodiment as confirmed based on a precisionstructural analysis by four-axial X-ray diffraction;

[0034]FIG. 5 is a graph showing relationship between temperature andelectrical resistance with respect to the MgB₂ single crystal accordingto the embodiment;

[0035]FIG. 6 is a graph showing relationship between temperature andmagnetic susceptibility with respect to the MgB₂ single crystalaccording to the embodiment; and

[0036]FIG. 7 is a graph showing temperature-dependency of a secondmagnetic field Hc₂ and that of an irreversible magnetic field H_(irr),found from electrical resistance measurement, respectively, with respectto the MgB₂ single crystal according to the embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

[0037] An embodiment of the invention together with operation thereof isdescribed in detail hereinafter.

[0038] An MgB₂ single crystal according to the invention has anisotropyin respect of its superconductive properties in a critical magneticfield and so forth, and by bonding together a plurality of such singlecrystals having anisotropy in respect of superconductive properties insuch a way that respective crystal orientations thereof are aligned witheach other, or by causing a plurality of such single crystals adjacentto each other to grow while controlling them in such a way thatrespective crystal orientations thereof are aligned with each other byuse of a seed crystal, it is possible to make up “a superconductorcapable of exhibiting anisotropic superconductive properties of MgB₂ inthe best state”.

[0039] Further, with the use of a method according to the invention, itis possible to obtain an MgB₂ single crystal exhibiting large anisotropyeven at a relatively high temperature such that “a critical magneticfield anisotropy ratio at a temperature of 25K is not less than 2.3”, sothat an MgB₂ superconductor quite excellent in superconductiveproperties can be obtained by properly bonding the MgB₂ single crystalstogether.

[0040] Further, with the use of the method according to the invention,it is possible in addition to the above-described characteristic toobtain the MgB₂ single crystal wherein, in the case where a magneticfield is applied thereto so as to be parallel with boron faces, anirreversible magnetic field exhibits a value very close to that for asecond magnetic field such that an irreversible magnetic field H_(irr),is equivalent to not less than 95% of a second magnetic field Hc₂ in thecase where the magnetic field is applied thereto so as to be parallelwith the boron faces, so that it also becomes possible to provide asuperconductor having a very wide region that allows current to flowtherethrough although this is conditional on the magnetic field beingapplied thereto so as to be parallel with the boron faces.

[0041] Now, as previously described, since MgB₂ is caused to bedecomposed into MgB₄ and MgB₆ before MgB₂ is melted if the same in anas-is state is heated at a high temperature, it has been impossible toobtain MgB₂ melt deemed necessary in order to cause growth of the MgB₂single crystal. However, if “a mixed raw material of Mg and B” or “aprecursor containing MgB₂ crystallites obtained by causing reaction ofthe mixed raw material of Mg and B”, kept in contact with hexagonalboron nitride (hBN), is heated under high pressure, this will generateMg₃BN₃ that is a eutectic composition of Mg, B and N, whereupon thereoccur regions where Mg₃BN₃ is turned into melt at a temperature lowerthan the decomposition temperature of MgB₂. Then, if the raw material iskept in the regions, there occurs crystallization of MgB₂ crystals fromthe melt of Mg₃BN₃, and crystal growth continues for relatively longhours without the MgB₂ crystals being decomposed into MgB₄ and so forth.

[0042] Thus, it becomes possible to obtain the MgB₂ single crystals, acase of which has not been reported in specific terms in the past.

[0043]FIG. 1 is a graph showing optimum conditions (temperature,pressure) for obtaining the MgB₂ single crystal, diagrammingrelationship between pressure, and the melting temperature of Mg, thedecomposition temperature of MgB₂, the melting temperature of Mg₃BN₃,and the temperature of phase transition from hBN (hexagonal BN) to cBN(cubic BN), respectively.

[0044] In FIG. 1, upon continuation of pressurization and heating of amixed raw material of Mg and B″, kept in contact with hexagonal boronnitride (hBN), there occurs generation of MgB₂ (in powdery form) inregions of relatively low temperature and low pressure due to reactionbetween Mg and B, however, such MgB₂ undergoes decomposition into MgB₄and MgB₆ in regions of low pressure when applied temperature is raised.Upon pressure reaching not lower than 3 GPa, and temperature reachingnot lower than 1300° C., however, a eutectic composition Mg₃BN₃ isgenerated in the presence of BN, thereby turning into a melt phase. Ifthe melt phase is maintained at a temperature lower than thedecomposition temperature of MgB₂, this will cause crystallization ofMgB₂ from the Mg₃BN₃ melt, and an MgB₂ crystal undergoes crystal growthwithout being decomposed into MgB₄ and MgB₆.

[0045] In the region of cBN, however, there occurs a state whereconcurrent growth of MgB₂ crystals and cBN crystals of a cubic systemproceeds, allowing existence of MgB₂ as an intermediate product in thecourse of transformation from Mg₃BN₃ to cBN. Accordingly, in ahigh-pressure region exceeding 6 GPa, reaction of transformation fromMg₃BN₃ to cBN proceeds without hardly any pause as is evident from FIG.1, so that actual work for such processing lacks practicality because aprocessing time range for taking out the MgB₂ crystals is extremelyshort.

[0046] For this reason, in the manufacture of the MgB₂ crystals, it ispreferable to adopt conditions such that mixed raw material powders ofMg and B are sealed in a high-pressure capsule made of BN, and areheated at a temperature in the range of 1300 to 1700° C. and under ahigh pressure in the range of 3 to 6 GPa (preferably 3.5 to 6 GPa).

[0047] In this case, if MgB₂ crystallites in non-melted state exist inMg₃BN₃ melt, the MgB₂ crystallites act as nuclei, promotingcrystallization and growth of MgB₂ crystals, and accordingly, it is alsorecommendable to adopt a process comprising steps of preparing aprecursor containing MgB₂ crystallites, obtained by causing reaction ofa mixed raw material of Mg and B in advance at an approximatelyintermediate temperature under an intermediate pressure (for example, at900° C. and under pressure in the range of about 2 to 4 GPa), andholding the precursor, in a state where it is kept in contact withhexagonal boron nitride (hBN), at a temperature in the range of 1300 to1700° C. and under a high pressure in the range of 3 to 6 GPa.

[0048] Further, it is deemed appropriate that an oxygen partial pressureshould be kept constant in the course of crystal growth because anequilibrium partial pressure of a B—N—Mg system is caused to undergo achange due to reaction of oxygen itself therewith, however, it isextremely difficult to prevent oxygen from being mixed in throughoutworks for mixing and sealing of raw materials, and so froth.Accordingly, a reducing agent such as Mg and so forth, and the mixed rawmaterial powders of Mg and B are preferably caused to coexist while bothare separated spatially from each other in an atmosphere of heating rawmaterials for manufacturing single crystals. The reducing agent that iscaused to coexist will react with a trace of oxygen mixed into theheating atmosphere, thereby fulfilling the function of assisting growthof the MgB₂ crystals.

[0049] Preferable as the reducing agent is a metallic material havingstrong oxidizability, such as Mg, etc. that will not be precipitated asan impurity since it is one of constituent elements of a target product.

[0050] Further, it is also recommendable to cause a temperature gradientof from 150 to 300° C. to occur in the Mg₃BN₃ melt occurring as a resultof heating and holding raw materials for manufacturing the MgB₂ singlecrystals, kept in contact with hexagonal boron nitride (hBN), whenheating and holding said raw materials. By so doing, crystal growth canbe promoted although there arise worries that crystal growth takeslonger time at a temperature below 150° C. of the temperature gradientwhile growth of large and high-purity single crystals becomes unstableat a temperature in excess of 300° C.

[0051] Needless to say, in providing the melt for crystal growth withthe temperature gradient, a temperature gradient that tends to occurunintentionally between the central part of a reactor and end parts, andso forth, thereof may be utilized as it is.

[0052] Incidentally, when forming a superconductive material forpractical application (for example, a superconducting wire rod) bytaking advantage of the properties of the above-described MgB₂ singlecrystals, other materials such as Mg, B, MgB₂ powders, etc. that havenot reacted as yet tend to be mixed in, however, it goes without sayingthat appropriate superconductive properties based on the MgB₂ singlecrystals can be obtained even if such materials are mixed in.

[0053] Further, it also goes without saying that appropriatesuperconductive properties can be obtained even if metal such as Ag, Cu,and so forth is used as a binder to facilitate formation of, forexample, a wire rod when forming the wire rod from an MgB₂ singlecrystal material.

[0054] Subsequently, the invention is described in more specific termswith reference to an embodiment thereof.

[0055] Embodiment

[0056] First, Mg powders of 100 mesh in grain size, and B powders(amorphous powders about 0.91 μm in grain diameter) were mixed with eachother so as to have an atomic ratio of “Mg: B=1:2”, and a mixture wascompression-molded into compacts each 5 mm in diameter, and 4 mm inthickness.

[0057] Further, separately from the compacts, there were prepared “BNpellets” and “compacts of Mg powders (reducing agent)”, equivalent indimensions to the compacts.

[0058] Subsequently, as shown in FIG. 2, these were sealed into acylindrical vessel (high-pressure vessel 5 mm in inside diameter and 10mm in length), made of hexagonal BN, in the atmosphere (BN pellets weredisposed at both ends of the vessel, respectively, doubling asrespective lids).

[0059] Then, in a first stage, in order to produce a precursorcontaining MgB₂ crystallites by causing reaction of mixed powders of Mgand B, pressure was applied to the interior of the vessel made of BN soas to reach 5 GPa while heating the vessel at 900° C. for 15 minutes.

[0060] Subsequently, the interior of the vessel made of BN, maintaininga pressurized condition at 5 GPa, was further heated up to a temperatureof 1500° C., which condition was held for 25 minutes, thereby attemptingto implement growth of MgB₂ single crystals.

[0061] Further, at this point in time, a temperature gradient of 210° C.between the central part of the vessel made of BN and the respectiveends thereof (the temperature gradient indicated by the respectivearrows in FIG. 2) was caused to occur in Mg₃BN₃ melt that was producedin the vessel made of BN.

[0062] Upon observation of material inside the vessel made of BN aftercompletion of such processing as described above with the use of anelectron microscope, it was confirmed that crystals each on the order of0.5 mm in size were obtained as shown in FIG. 3, and the crystals wereMgB₂ single crystals of a hexagonal system structure as shown in FIG. 4as a result of a precision structural analysis conducted by X-raydiffraction.

[0063]FIGS. 5 and 6 show relationship between temperature and electricalresistance, and relationship between temperature and a magneticsusceptibility ratio, based on the results of examination on the MgB₂single crystals, respectively, and it can be confirmed from any of thefigures that transition to superconductivity occurs at around 38K.

[0064] Further, with reference to the MgB₂ single crystal,temperature-dependency of a second magnetic field Hc₂ and that of anirreversible magnetic field H_(irr), found from electrical resistancemeasurement, respectively, were sorted out, and are shown in FIG. 7.

[0065] As shown FIG. 7, the temperature-dependency of the secondmagnetic field Hc₂ and that of the irreversible magnetic field H_(irr),in the case (B//ab) where a magnetic field is applied in such a way asto be parallel with boron faces, respectively, differ considerably fromthose in the case (B//c) where the magnetic field is applied in such away as to be perpendicular to the boron faces, and at 25K, an anisotropyratio of the second magnetic field Hc₂, becomes 2.3 or greater.

[0066] Now, with reference to the MgB₂ single crystal, there has beenobserved a feature in that when a magnetic field is applied in such away as to be parallel with the boron faces (H//ab), the irreversiblemagnetic field H_(irr) is extremely close to the second magnetic fieldHc₂ (H_(irr)≈Hc₂) (refer to FIG. 7). In this connection, it is to bepointed out that, in the case where the magnetic field is applied insuch a way as to be perpendicular to the boron faces (H//c), H_(irr) issubstantially equivalent to only half of Hc₂.

[0067] A superconductor generally maintains a superconductive state in amagnetic field whose strength is less than that of a second magneticfield Hc₂, however, in a magnetic field whose strength is grater thanthat of an irreversible magnetic field H_(irr), there occurs flux motioninside the superconductor, thereby causing generation of resistance, sothat flow of superconducting current is stopped. In other words, itfollows that a region where current can flow is very wide if theirreversible magnetic field H_(irr) is extremely close to the secondmagnetic field Hc₂ (H_(irr)≈Hc₂).

[0068] Thus, the MgB₂ single crystal is an extremely advantageoussuperconductive material firm an industrial point of view that can allowlarge superconducting current to flow even if a high magnetic filed isapplied thereto provided that the high magnetic filed applied isparallel with the boron faces.

[0069] Indutrial Applicability

[0070] The invention can not only stably supply an MgB₂ single crystalhaving a specific anisotropy in its superconductive properties, and canmanufacture a superconductive material excellent in superconductiveproperties, containing the MgB₂ single crystal, but also contribute toproviding useful information for selection of a process formanufacturing MgB₂ superconducting wire rods and thin films.Accordingly, the invention can make great contribution to expansion ofapplication for MgB₂ that is highly hoped for as a superconductivematerial having high utilization.

1. An MgB₂ single crystal having anisotropic superconductive propertiessuch that a critical magnetic field anisotropy ratio at a temperature of25K is not less than 2.3, and in the case where a magnetic field isapplied thereto so as to be parallel with boron faces, an irreversiblemagnetic field strength is equivalent to not less than 95% of a secondmagnetic field strength.
 2. A method of manufacturing MgB₂ singlecrystals, comprising the steps of: preparing a mixed raw material of Mgand B; heating and melting the mixed raw material, kept in contact withboron nitride (hBN), at a high temperature in the range of 1300 to 1700°C. and under a high pressure in the range of 3 to 6 GPa; and causinggrowth of the MgB₂ single crystals having anisotropic superconductiveproperties by holding the mixed raw material in the above-describedstate.
 3. A method of manufacturing MgB₂ single crystals, comprising thesteps of: producing a precursor containing MgB₂ crystallites, obtainedby causing reaction of a mixed raw material of Mg and B; heating andmelting the precursor, kept in contact with hexagonal boron nitride(hBN), at a high temperature in the range of 1300 to 1700° C. and undera high pressure in the range of 3 to 6 GPa; and causing growth of theMgB₂ single crystals having anisotropic superconductive properties byholding the precursor in the above-described state.
 4. The method ofmanufacturing MgB₂ single crystals, according to claim 2 or claim 3,wherein in the course of heating, and melting the raw material or theprecursor, to be heated and melted, at the high temperature and underthe high pressure, and holding the same in the above-described state, areducing agent is caused to coexist therewith.
 5. The method ofmanufacturing MgB₂ single crystals, according to any of claims 2 to 4,wherein in the course of heating and melting the raw material or theprecursor, to be heated and melted, at the high temperature and underthe high pressure, and holding the same in the above-described state, atemperature gradient of from 150 to 300° C. is provided in meltoccurring as a result of the heating and melting the raw material or theprecursor.
 6. The method of manufacturing MgB₂ single crystals,according to any of claims 2 to 5, wherein the MgB₂ single crystals haveanisotropic superconductive properties which are superconductiveproperties such that a critical magnetic field anisotropy ratio at atemperature of 25K is not less than 2.3, and in the case where amagnetic field is applied thereto so as to be parallel with boron faces,an irreversible magnetic field strength is equivalent to not less than95% of a second magnetic field strength.
 7. A superconductive materialcomprising MgB₂ single crystals having anisotropic superconductiveproperties such that a critical magnetic field anisotropy ratio at atemperature of 25K is not less than 2.3, and in the case where amagnetic field is applied thereto so as to be parallel with boron faces,an irreversible magnetic field strength is equivalent to not less than95% of a second magnetic field strength.
 8. A superconductive wire rodcomprising MgB₂ single crystals having anisotropic superconductiveproperties such that a critical magnetic field anisotropy ratio at atemperature of 25K is not less than 2.3, and in the case where amagnetic field is applied thereto so as to be parallel with boron faces,an irreversible magnetic field strength is equivalent to not less than95% of a second magnetic field strength.